Spread the solution out on the grout lines. Let it sit for a few minutes, and then scrub it with the tile brush. A grout brush is a great tool for this because they have extra stiff bristles, but if you don't have a grout brush, an old toothbrush is a great substitute.
It's kind of funny that this is only one step because it's going to take you way longer than the rest of the process. You're going to have to really, truly scrub. If you're working on an entire room, expect your arms to be sore. Also, be sure to start at the back of the room and work your way to an exit; no scrubbing yourself into a corner, okay?